In all our focus on weight, waist size, calorie count and other measurements, it can be easy to forget one of the most important – Body Mass Index (BMI). Popularized in the late 1990s by exercise scientists and physicians, the formula and idea of Body Mass Index originated in the mid 1800s with Belgian statistician Adolphe Quetelet.
The theory was simple: the density of one’s body in proportion to height and weight was a more accurate indicator of health than simply weight alone. It would take several decades for the scientific community to corroborate the idea with facts like muscle being approximately four times as dense as fat (meaning that a 1-pound block of muscle will take up ¼ the space of a 1-pound block of fat). This is why you may find that very slim-looking athletes weight surprisingly more than you might expect. BMI also helps explain why a 6-foot 4-inch man can be fit and trim at 190 pounds, but the same weight on a 5-foot 8-inch man makes him appear chunky.
Calculated by dividing weight (in pounds or kilograms) by height (in inches squared or meters squared), BMI yields a two-digit score generally ranging between 15 and 45. Low scores like 18.5 and lower are considered to indicate an individual is underweight for their size. Between 18.5 and 24.5 is typically regarded as “normal” or healthy weight range for a person’s size. Scores between 25 and 29 are associated with being moderately overweight. And scores of 30 or above are connected with obesity. Other countries use different ranges to define these categories, but only vary slightly.
What you should know about your BMI and how you can use it as a tool in your own health and wellness is how accurate it can be. While it can be easy to ignore four or five pounds on the scale, BMI measures against your height, which is constant. So the numbers shift slightly with any level of weight gain. However, this means you must pay attention to the full number – both before and after the decimal point. Because the ranges between normal weight and overweight are only 6 digits, every little bit counts. At the same time, if you find you fixate on your weight too much, you may appreciate the mild amount of forgiveness that focusing on BMI allows you. In this way, BMI can be very healthy from a mental perspective since it is much more individualized than comparing pounds with friends, training partners or celebrities.